1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Shareable mutable containers.
13 //! Rust memory safety is based on this rule: Given an object `T`, it is only possible to
14 //! have one of the following:
16 //! - Having several immutable references (`&T`) to the object (also known as **aliasing**).
17 //! - Having one mutable reference (`&mut T`) to the object (also known as **mutability**).
19 //! This is enforced by the Rust compiler. However, there are situations where this rule is not
20 //! flexible enough. Sometimes it is required to have multiple references to an object and yet
23 //! Shareable mutable containers exist to permit mutability in a controlled manner, even in the
24 //! presence of aliasing. Both `Cell<T>` and `RefCell<T>` allows to do this in a single threaded
25 //! way. However, neither `Cell<T>` nor `RefCell<T>` are thread safe (they do not implement
26 //! `Sync`). If you need to do aliasing and mutation between multiple threads it is possible to
27 //! use [`Mutex`](../../std/sync/struct.Mutex.html),
28 //! [`RwLock`](../../std/sync/struct.RwLock.html) or
29 //! [`atomic`](../../core/sync/atomic/index.html) types.
31 //! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e.
32 //! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`)
33 //! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast
34 //! with typical Rust types that exhibit 'inherited mutability'.
36 //! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior
37 //! mutability by moving values in and out of the `Cell<T>`. To use references instead of values,
38 //! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides
39 //! methods to retrieve and change the current interior value:
41 //! - For types that implement `Copy`, the `get` method retrieves the current interior value.
42 //! - For types that implement `Default`, the `take` method replaces the current interior value
43 //! with `Default::default()` and returns the replaced value.
44 //! - For all types, the `replace` method replaces the current interior value and returns the
45 //! replaced value and the `into_inner` method consumes the `Cell<T>` and returns the interior
46 //! value. Additionally, the `set` method replaces the interior value, dropping the replaced
49 //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
50 //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
51 //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
52 //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
53 //! to borrow a value that is already mutably borrowed; when this happens it results in thread
56 //! # When to choose interior mutability
58 //! The more common inherited mutability, where one must have unique access to mutate a value, is
59 //! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
60 //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
61 //! interior mutability is something of a last resort. Since cell types enable mutation where it
62 //! would otherwise be disallowed though, there are occasions when interior mutability might be
63 //! appropriate, or even *must* be used, e.g.
65 //! * Introducing mutability 'inside' of something immutable
66 //! * Implementation details of logically-immutable methods.
67 //! * Mutating implementations of `Clone`.
69 //! ## Introducing mutability 'inside' of something immutable
71 //! Many shared smart pointer types, including `Rc<T>` and `Arc<T>`, provide containers that can be
72 //! cloned and shared between multiple parties. Because the contained values may be
73 //! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
74 //! impossible to mutate data inside of these smart pointers at all.
76 //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
80 //! use std::collections::HashMap;
81 //! use std::cell::RefCell;
85 //! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
86 //! shared_map.borrow_mut().insert("africa", 92388);
87 //! shared_map.borrow_mut().insert("kyoto", 11837);
88 //! shared_map.borrow_mut().insert("piccadilly", 11826);
89 //! shared_map.borrow_mut().insert("marbles", 38);
93 //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
94 //! scenarios. Consider using `RwLock<T>` or `Mutex<T>` if you need shared mutability in a
95 //! multi-threaded situation.
97 //! ## Implementation details of logically-immutable methods
99 //! Occasionally it may be desirable not to expose in an API that there is mutation happening
100 //! "under the hood". This may be because logically the operation is immutable, but e.g. caching
101 //! forces the implementation to perform mutation; or because you must employ mutation to implement
102 //! a trait method that was originally defined to take `&self`.
105 //! # #![allow(dead_code)]
106 //! use std::cell::RefCell;
109 //! edges: Vec<(i32, i32)>,
110 //! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
114 //! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
115 //! // Create a new scope to contain the lifetime of the
116 //! // dynamic borrow
118 //! // Take a reference to the inside of cache cell
119 //! let mut cache = self.span_tree_cache.borrow_mut();
120 //! if cache.is_some() {
121 //! return cache.as_ref().unwrap().clone();
124 //! let span_tree = self.calc_span_tree();
125 //! *cache = Some(span_tree);
128 //! // Recursive call to return the just-cached value.
129 //! // Note that if we had not let the previous borrow
130 //! // of the cache fall out of scope then the subsequent
131 //! // recursive borrow would cause a dynamic thread panic.
132 //! // This is the major hazard of using `RefCell`.
133 //! self.minimum_spanning_tree()
135 //! # fn calc_span_tree(&self) -> Vec<(i32, i32)> { vec![] }
139 //! ## Mutating implementations of `Clone`
141 //! This is simply a special - but common - case of the previous: hiding mutability for operations
142 //! that appear to be immutable. The `clone` method is expected to not change the source value, and
143 //! is declared to take `&self`, not `&mut self`. Therefore any mutation that happens in the
144 //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a
148 //! #![feature(core_intrinsics)]
149 //! use std::cell::Cell;
150 //! use std::ptr::NonNull;
151 //! use std::intrinsics::abort;
153 //! struct Rc<T: ?Sized> {
154 //! ptr: NonNull<RcBox<T>>
157 //! struct RcBox<T: ?Sized> {
158 //! strong: Cell<usize>,
159 //! refcount: Cell<usize>,
163 //! impl<T: ?Sized> Clone for Rc<T> {
164 //! fn clone(&self) -> Rc<T> {
165 //! self.inc_strong();
166 //! Rc { ptr: self.ptr }
170 //! trait RcBoxPtr<T: ?Sized> {
172 //! fn inner(&self) -> &RcBox<T>;
174 //! fn strong(&self) -> usize {
175 //! self.inner().strong.get()
178 //! fn inc_strong(&self) {
181 //! .set(self.strong()
183 //! .unwrap_or_else(|| unsafe { abort() }));
187 //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
188 //! fn inner(&self) -> &RcBox<T> {
190 //! self.ptr.as_ref()
197 #![stable(feature = "rust1", since = "1.0.0")]
200 use fmt::{self, Debug, Display};
203 use ops::{Deref, DerefMut, CoerceUnsized};
206 /// A mutable memory location.
210 /// Here you can see how using `Cell<T>` allows to use mutable field inside
211 /// immutable struct (which is also called 'interior mutability').
214 /// use std::cell::Cell;
216 /// struct SomeStruct {
217 /// regular_field: u8,
218 /// special_field: Cell<u8>,
221 /// let my_struct = SomeStruct {
222 /// regular_field: 0,
223 /// special_field: Cell::new(1),
226 /// let new_value = 100;
228 /// // ERROR, because my_struct is immutable
229 /// // my_struct.regular_field = new_value;
231 /// // WORKS, although `my_struct` is immutable, field `special_field` is mutable because it is Cell
232 /// my_struct.special_field.set(new_value);
233 /// assert_eq!(my_struct.special_field.get(), new_value);
236 /// See the [module-level documentation](index.html) for more.
237 #[stable(feature = "rust1", since = "1.0.0")]
239 value: UnsafeCell<T>,
242 impl<T:Copy> Cell<T> {
243 /// Returns a copy of the contained value.
248 /// use std::cell::Cell;
250 /// let c = Cell::new(5);
252 /// let five = c.get();
255 #[stable(feature = "rust1", since = "1.0.0")]
256 pub fn get(&self) -> T {
257 unsafe{ *self.value.get() }
260 /// Updates the contained value using a function and returns the new value.
265 /// #![feature(cell_update)]
267 /// use std::cell::Cell;
269 /// let c = Cell::new(5);
270 /// let new = c.update(|x| x + 1);
272 /// assert_eq!(new, 6);
273 /// assert_eq!(c.get(), 6);
276 #[unstable(feature = "cell_update", issue = "50186")]
277 pub fn update<F>(&self, f: F) -> T
281 let old = self.get();
288 #[stable(feature = "rust1", since = "1.0.0")]
289 unsafe impl<T> Send for Cell<T> where T: Send {}
291 #[stable(feature = "rust1", since = "1.0.0")]
292 impl<T> !Sync for Cell<T> {}
294 #[stable(feature = "rust1", since = "1.0.0")]
295 impl<T:Copy> Clone for Cell<T> {
297 fn clone(&self) -> Cell<T> {
298 Cell::new(self.get())
302 #[stable(feature = "rust1", since = "1.0.0")]
303 impl<T:Default> Default for Cell<T> {
304 /// Creates a `Cell<T>`, with the `Default` value for T.
306 fn default() -> Cell<T> {
307 Cell::new(Default::default())
311 #[stable(feature = "rust1", since = "1.0.0")]
312 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
314 fn eq(&self, other: &Cell<T>) -> bool {
315 self.get() == other.get()
319 #[stable(feature = "cell_eq", since = "1.2.0")]
320 impl<T:Eq + Copy> Eq for Cell<T> {}
322 #[stable(feature = "cell_ord", since = "1.10.0")]
323 impl<T:PartialOrd + Copy> PartialOrd for Cell<T> {
325 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
326 self.get().partial_cmp(&other.get())
330 fn lt(&self, other: &Cell<T>) -> bool {
331 self.get() < other.get()
335 fn le(&self, other: &Cell<T>) -> bool {
336 self.get() <= other.get()
340 fn gt(&self, other: &Cell<T>) -> bool {
341 self.get() > other.get()
345 fn ge(&self, other: &Cell<T>) -> bool {
346 self.get() >= other.get()
350 #[stable(feature = "cell_ord", since = "1.10.0")]
351 impl<T:Ord + Copy> Ord for Cell<T> {
353 fn cmp(&self, other: &Cell<T>) -> Ordering {
354 self.get().cmp(&other.get())
358 #[stable(feature = "cell_from", since = "1.12.0")]
359 impl<T> From<T> for Cell<T> {
360 fn from(t: T) -> Cell<T> {
366 /// Creates a new `Cell` containing the given value.
371 /// use std::cell::Cell;
373 /// let c = Cell::new(5);
375 #[stable(feature = "rust1", since = "1.0.0")]
377 pub const fn new(value: T) -> Cell<T> {
379 value: UnsafeCell::new(value),
383 /// Returns a raw pointer to the underlying data in this cell.
388 /// use std::cell::Cell;
390 /// let c = Cell::new(5);
392 /// let ptr = c.as_ptr();
395 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
396 pub fn as_ptr(&self) -> *mut T {
400 /// Returns a mutable reference to the underlying data.
402 /// This call borrows `Cell` mutably (at compile-time) which guarantees
403 /// that we possess the only reference.
408 /// use std::cell::Cell;
410 /// let mut c = Cell::new(5);
411 /// *c.get_mut() += 1;
413 /// assert_eq!(c.get(), 6);
416 #[stable(feature = "cell_get_mut", since = "1.11.0")]
417 pub fn get_mut(&mut self) -> &mut T {
419 &mut *self.value.get()
423 /// Sets the contained value.
428 /// use std::cell::Cell;
430 /// let c = Cell::new(5);
435 #[stable(feature = "rust1", since = "1.0.0")]
436 pub fn set(&self, val: T) {
437 let old = self.replace(val);
441 /// Swaps the values of two Cells.
442 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
447 /// use std::cell::Cell;
449 /// let c1 = Cell::new(5i32);
450 /// let c2 = Cell::new(10i32);
452 /// assert_eq!(10, c1.get());
453 /// assert_eq!(5, c2.get());
456 #[stable(feature = "move_cell", since = "1.17.0")]
457 pub fn swap(&self, other: &Self) {
458 if ptr::eq(self, other) {
462 ptr::swap(self.value.get(), other.value.get());
466 /// Replaces the contained value, and returns it.
471 /// use std::cell::Cell;
473 /// let cell = Cell::new(5);
474 /// assert_eq!(cell.get(), 5);
475 /// assert_eq!(cell.replace(10), 5);
476 /// assert_eq!(cell.get(), 10);
478 #[stable(feature = "move_cell", since = "1.17.0")]
479 pub fn replace(&self, val: T) -> T {
480 mem::replace(unsafe { &mut *self.value.get() }, val)
483 /// Unwraps the value.
488 /// use std::cell::Cell;
490 /// let c = Cell::new(5);
491 /// let five = c.into_inner();
493 /// assert_eq!(five, 5);
495 #[stable(feature = "move_cell", since = "1.17.0")]
496 pub fn into_inner(self) -> T {
497 self.value.into_inner()
501 impl<T: Default> Cell<T> {
502 /// Takes the value of the cell, leaving `Default::default()` in its place.
507 /// use std::cell::Cell;
509 /// let c = Cell::new(5);
510 /// let five = c.take();
512 /// assert_eq!(five, 5);
513 /// assert_eq!(c.into_inner(), 0);
515 #[stable(feature = "move_cell", since = "1.17.0")]
516 pub fn take(&self) -> T {
517 self.replace(Default::default())
521 #[unstable(feature = "coerce_unsized", issue = "27732")]
522 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
524 /// A mutable memory location with dynamically checked borrow rules
526 /// See the [module-level documentation](index.html) for more.
527 #[stable(feature = "rust1", since = "1.0.0")]
528 pub struct RefCell<T: ?Sized> {
529 borrow: Cell<BorrowFlag>,
530 value: UnsafeCell<T>,
533 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
534 #[stable(feature = "try_borrow", since = "1.13.0")]
535 pub struct BorrowError {
539 #[stable(feature = "try_borrow", since = "1.13.0")]
540 impl Debug for BorrowError {
541 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
542 f.debug_struct("BorrowError").finish()
546 #[stable(feature = "try_borrow", since = "1.13.0")]
547 impl Display for BorrowError {
548 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
549 Display::fmt("already mutably borrowed", f)
553 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
554 #[stable(feature = "try_borrow", since = "1.13.0")]
555 pub struct BorrowMutError {
559 #[stable(feature = "try_borrow", since = "1.13.0")]
560 impl Debug for BorrowMutError {
561 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
562 f.debug_struct("BorrowMutError").finish()
566 #[stable(feature = "try_borrow", since = "1.13.0")]
567 impl Display for BorrowMutError {
568 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
569 Display::fmt("already borrowed", f)
573 // Positive values represent the number of `Ref` active. Negative values
574 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
575 // active at a time if they refer to distinct, nonoverlapping components of a
576 // `RefCell` (e.g., different ranges of a slice).
578 // `Ref` and `RefMut` are both two words in size, and so there will likely never
579 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
580 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
581 // However, this is not a guarantee, as a pathological program could repeatedly
582 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
583 // explicitly check for overflow and underflow in order to avoid unsafety, or at
584 // least behave correctly in the event that overflow or underflow happens (e.g.,
585 // see BorrowRef::new).
586 type BorrowFlag = isize;
587 const UNUSED: BorrowFlag = 0;
590 fn is_writing(x: BorrowFlag) -> bool {
595 fn is_reading(x: BorrowFlag) -> bool {
600 /// Creates a new `RefCell` containing `value`.
605 /// use std::cell::RefCell;
607 /// let c = RefCell::new(5);
609 #[stable(feature = "rust1", since = "1.0.0")]
611 pub const fn new(value: T) -> RefCell<T> {
613 value: UnsafeCell::new(value),
614 borrow: Cell::new(UNUSED),
618 /// Consumes the `RefCell`, returning the wrapped value.
623 /// use std::cell::RefCell;
625 /// let c = RefCell::new(5);
627 /// let five = c.into_inner();
629 #[stable(feature = "rust1", since = "1.0.0")]
631 pub fn into_inner(self) -> T {
632 // Since this function takes `self` (the `RefCell`) by value, the
633 // compiler statically verifies that it is not currently borrowed.
634 // Therefore the following assertion is just a `debug_assert!`.
635 debug_assert!(self.borrow.get() == UNUSED);
636 self.value.into_inner()
639 /// Replaces the wrapped value with a new one, returning the old value,
640 /// without deinitializing either one.
642 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
646 /// Panics if the value is currently borrowed.
651 /// use std::cell::RefCell;
652 /// let cell = RefCell::new(5);
653 /// let old_value = cell.replace(6);
654 /// assert_eq!(old_value, 5);
655 /// assert_eq!(cell, RefCell::new(6));
658 #[stable(feature = "refcell_replace", since="1.24.0")]
659 pub fn replace(&self, t: T) -> T {
660 mem::replace(&mut *self.borrow_mut(), t)
663 /// Replaces the wrapped value with a new one computed from `f`, returning
664 /// the old value, without deinitializing either one.
666 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
670 /// Panics if the value is currently borrowed.
675 /// #![feature(refcell_replace_swap)]
676 /// use std::cell::RefCell;
677 /// let cell = RefCell::new(5);
678 /// let old_value = cell.replace_with(|&mut old| old + 1);
679 /// assert_eq!(old_value, 5);
680 /// assert_eq!(cell, RefCell::new(6));
683 #[unstable(feature = "refcell_replace_swap", issue="43570")]
684 pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
685 let mut_borrow = &mut *self.borrow_mut();
686 let replacement = f(mut_borrow);
687 mem::replace(mut_borrow, replacement)
690 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
691 /// without deinitializing either one.
693 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
697 /// Panics if the value in either `RefCell` is currently borrowed.
702 /// use std::cell::RefCell;
703 /// let c = RefCell::new(5);
704 /// let d = RefCell::new(6);
706 /// assert_eq!(c, RefCell::new(6));
707 /// assert_eq!(d, RefCell::new(5));
710 #[stable(feature = "refcell_swap", since="1.24.0")]
711 pub fn swap(&self, other: &Self) {
712 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
716 impl<T: ?Sized> RefCell<T> {
717 /// Immutably borrows the wrapped value.
719 /// The borrow lasts until the returned `Ref` exits scope. Multiple
720 /// immutable borrows can be taken out at the same time.
724 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
725 /// [`try_borrow`](#method.try_borrow).
730 /// use std::cell::RefCell;
732 /// let c = RefCell::new(5);
734 /// let borrowed_five = c.borrow();
735 /// let borrowed_five2 = c.borrow();
738 /// An example of panic:
741 /// use std::cell::RefCell;
744 /// let result = thread::spawn(move || {
745 /// let c = RefCell::new(5);
746 /// let m = c.borrow_mut();
748 /// let b = c.borrow(); // this causes a panic
751 /// assert!(result.is_err());
753 #[stable(feature = "rust1", since = "1.0.0")]
755 pub fn borrow(&self) -> Ref<T> {
756 self.try_borrow().expect("already mutably borrowed")
759 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
762 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
763 /// taken out at the same time.
765 /// This is the non-panicking variant of [`borrow`](#method.borrow).
770 /// use std::cell::RefCell;
772 /// let c = RefCell::new(5);
775 /// let m = c.borrow_mut();
776 /// assert!(c.try_borrow().is_err());
780 /// let m = c.borrow();
781 /// assert!(c.try_borrow().is_ok());
784 #[stable(feature = "try_borrow", since = "1.13.0")]
786 pub fn try_borrow(&self) -> Result<Ref<T>, BorrowError> {
787 match BorrowRef::new(&self.borrow) {
789 value: unsafe { &*self.value.get() },
792 None => Err(BorrowError { _private: () }),
796 /// Mutably borrows the wrapped value.
798 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
799 /// from it exit scope. The value cannot be borrowed while this borrow is
804 /// Panics if the value is currently borrowed. For a non-panicking variant, use
805 /// [`try_borrow_mut`](#method.try_borrow_mut).
810 /// use std::cell::RefCell;
812 /// let c = RefCell::new(5);
814 /// *c.borrow_mut() = 7;
816 /// assert_eq!(*c.borrow(), 7);
819 /// An example of panic:
822 /// use std::cell::RefCell;
825 /// let result = thread::spawn(move || {
826 /// let c = RefCell::new(5);
827 /// let m = c.borrow();
829 /// let b = c.borrow_mut(); // this causes a panic
832 /// assert!(result.is_err());
834 #[stable(feature = "rust1", since = "1.0.0")]
836 pub fn borrow_mut(&self) -> RefMut<T> {
837 self.try_borrow_mut().expect("already borrowed")
840 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
842 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
843 /// from it exit scope. The value cannot be borrowed while this borrow is
846 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
851 /// use std::cell::RefCell;
853 /// let c = RefCell::new(5);
856 /// let m = c.borrow();
857 /// assert!(c.try_borrow_mut().is_err());
860 /// assert!(c.try_borrow_mut().is_ok());
862 #[stable(feature = "try_borrow", since = "1.13.0")]
864 pub fn try_borrow_mut(&self) -> Result<RefMut<T>, BorrowMutError> {
865 match BorrowRefMut::new(&self.borrow) {
866 Some(b) => Ok(RefMut {
867 value: unsafe { &mut *self.value.get() },
870 None => Err(BorrowMutError { _private: () }),
874 /// Returns a raw pointer to the underlying data in this cell.
879 /// use std::cell::RefCell;
881 /// let c = RefCell::new(5);
883 /// let ptr = c.as_ptr();
886 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
887 pub fn as_ptr(&self) -> *mut T {
891 /// Returns a mutable reference to the underlying data.
893 /// This call borrows `RefCell` mutably (at compile-time) so there is no
894 /// need for dynamic checks.
896 /// However be cautious: this method expects `self` to be mutable, which is
897 /// generally not the case when using a `RefCell`. Take a look at the
898 /// [`borrow_mut`] method instead if `self` isn't mutable.
900 /// Also, please be aware that this method is only for special circumstances and is usually
901 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
903 /// [`borrow_mut`]: #method.borrow_mut
908 /// use std::cell::RefCell;
910 /// let mut c = RefCell::new(5);
911 /// *c.get_mut() += 1;
913 /// assert_eq!(c, RefCell::new(6));
916 #[stable(feature = "cell_get_mut", since = "1.11.0")]
917 pub fn get_mut(&mut self) -> &mut T {
919 &mut *self.value.get()
924 #[stable(feature = "rust1", since = "1.0.0")]
925 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
927 #[stable(feature = "rust1", since = "1.0.0")]
928 impl<T: ?Sized> !Sync for RefCell<T> {}
930 #[stable(feature = "rust1", since = "1.0.0")]
931 impl<T: Clone> Clone for RefCell<T> {
934 /// Panics if the value is currently mutably borrowed.
936 fn clone(&self) -> RefCell<T> {
937 RefCell::new(self.borrow().clone())
941 #[stable(feature = "rust1", since = "1.0.0")]
942 impl<T:Default> Default for RefCell<T> {
943 /// Creates a `RefCell<T>`, with the `Default` value for T.
945 fn default() -> RefCell<T> {
946 RefCell::new(Default::default())
950 #[stable(feature = "rust1", since = "1.0.0")]
951 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
954 /// Panics if the value in either `RefCell` is currently borrowed.
956 fn eq(&self, other: &RefCell<T>) -> bool {
957 *self.borrow() == *other.borrow()
961 #[stable(feature = "cell_eq", since = "1.2.0")]
962 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
964 #[stable(feature = "cell_ord", since = "1.10.0")]
965 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
968 /// Panics if the value in either `RefCell` is currently borrowed.
970 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
971 self.borrow().partial_cmp(&*other.borrow())
976 /// Panics if the value in either `RefCell` is currently borrowed.
978 fn lt(&self, other: &RefCell<T>) -> bool {
979 *self.borrow() < *other.borrow()
984 /// Panics if the value in either `RefCell` is currently borrowed.
986 fn le(&self, other: &RefCell<T>) -> bool {
987 *self.borrow() <= *other.borrow()
992 /// Panics if the value in either `RefCell` is currently borrowed.
994 fn gt(&self, other: &RefCell<T>) -> bool {
995 *self.borrow() > *other.borrow()
1000 /// Panics if the value in either `RefCell` is currently borrowed.
1002 fn ge(&self, other: &RefCell<T>) -> bool {
1003 *self.borrow() >= *other.borrow()
1007 #[stable(feature = "cell_ord", since = "1.10.0")]
1008 impl<T: ?Sized + Ord> Ord for RefCell<T> {
1011 /// Panics if the value in either `RefCell` is currently borrowed.
1013 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1014 self.borrow().cmp(&*other.borrow())
1018 #[stable(feature = "cell_from", since = "1.12.0")]
1019 impl<T> From<T> for RefCell<T> {
1020 fn from(t: T) -> RefCell<T> {
1025 #[unstable(feature = "coerce_unsized", issue = "27732")]
1026 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1028 struct BorrowRef<'b> {
1029 borrow: &'b Cell<BorrowFlag>,
1032 impl<'b> BorrowRef<'b> {
1034 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1035 let b = borrow.get();
1036 if is_writing(b) || b == isize::max_value() {
1037 // If there's currently a writing borrow, or if incrementing the
1038 // refcount would overflow into a writing borrow.
1042 Some(BorrowRef { borrow })
1047 impl<'b> Drop for BorrowRef<'b> {
1049 fn drop(&mut self) {
1050 let borrow = self.borrow.get();
1051 debug_assert!(is_reading(borrow));
1052 self.borrow.set(borrow - 1);
1056 impl<'b> Clone for BorrowRef<'b> {
1058 fn clone(&self) -> BorrowRef<'b> {
1059 // Since this Ref exists, we know the borrow flag
1060 // is a reading borrow.
1061 let borrow = self.borrow.get();
1062 debug_assert!(is_reading(borrow));
1063 // Prevent the borrow counter from overflowing into
1064 // a writing borrow.
1065 assert!(borrow != isize::max_value());
1066 self.borrow.set(borrow + 1);
1067 BorrowRef { borrow: self.borrow }
1071 /// Wraps a borrowed reference to a value in a `RefCell` box.
1072 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1074 /// See the [module-level documentation](index.html) for more.
1075 #[stable(feature = "rust1", since = "1.0.0")]
1076 pub struct Ref<'b, T: ?Sized + 'b> {
1078 borrow: BorrowRef<'b>,
1081 #[stable(feature = "rust1", since = "1.0.0")]
1082 impl<'b, T: ?Sized> Deref for Ref<'b, T> {
1086 fn deref(&self) -> &T {
1091 impl<'b, T: ?Sized> Ref<'b, T> {
1094 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1096 /// This is an associated function that needs to be used as
1097 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1098 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1100 #[stable(feature = "cell_extras", since = "1.15.0")]
1102 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1105 borrow: orig.borrow.clone(),
1109 /// Make a new `Ref` for a component of the borrowed data.
1111 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1113 /// This is an associated function that needs to be used as `Ref::map(...)`.
1114 /// A method would interfere with methods of the same name on the contents
1115 /// of a `RefCell` used through `Deref`.
1120 /// use std::cell::{RefCell, Ref};
1122 /// let c = RefCell::new((5, 'b'));
1123 /// let b1: Ref<(u32, char)> = c.borrow();
1124 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1125 /// assert_eq!(*b2, 5)
1127 #[stable(feature = "cell_map", since = "1.8.0")]
1129 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1130 where F: FnOnce(&T) -> &U
1133 value: f(orig.value),
1134 borrow: orig.borrow,
1138 /// Split a `Ref` into multiple `Ref`s for different components of the
1141 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1143 /// This is an associated function that needs to be used as
1144 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1145 /// name on the contents of a `RefCell` used through `Deref`.
1150 /// #![feature(refcell_map_split)]
1151 /// use std::cell::{Ref, RefCell};
1153 /// let cell = RefCell::new([1, 2, 3, 4]);
1154 /// let borrow = cell.borrow();
1155 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1156 /// assert_eq!(*begin, [1, 2]);
1157 /// assert_eq!(*end, [3, 4]);
1159 #[unstable(feature = "refcell_map_split", issue = "51476")]
1161 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1162 where F: FnOnce(&T) -> (&U, &V)
1164 let (a, b) = f(orig.value);
1165 let borrow = orig.borrow.clone();
1166 (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
1170 #[unstable(feature = "coerce_unsized", issue = "27732")]
1171 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1173 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1174 impl<'a, T: ?Sized + fmt::Display> fmt::Display for Ref<'a, T> {
1175 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1180 impl<'b, T: ?Sized> RefMut<'b, T> {
1181 /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum
1184 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1186 /// This is an associated function that needs to be used as
1187 /// `RefMut::map(...)`. A method would interfere with methods of the same
1188 /// name on the contents of a `RefCell` used through `Deref`.
1193 /// use std::cell::{RefCell, RefMut};
1195 /// let c = RefCell::new((5, 'b'));
1197 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1198 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1199 /// assert_eq!(*b2, 5);
1202 /// assert_eq!(*c.borrow(), (42, 'b'));
1204 #[stable(feature = "cell_map", since = "1.8.0")]
1206 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1207 where F: FnOnce(&mut T) -> &mut U
1209 // FIXME(nll-rfc#40): fix borrow-check
1210 let RefMut { value, borrow } = orig;
1217 /// Split a `RefMut` into multiple `RefMut`s for different components of the
1220 /// The underlying `RefCell` will remain mutably borrowed until both
1221 /// returned `RefMut`s go out of scope.
1223 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1225 /// This is an associated function that needs to be used as
1226 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1227 /// same name on the contents of a `RefCell` used through `Deref`.
1232 /// #![feature(refcell_map_split)]
1233 /// use std::cell::{RefCell, RefMut};
1235 /// let cell = RefCell::new([1, 2, 3, 4]);
1236 /// let borrow = cell.borrow_mut();
1237 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1238 /// assert_eq!(*begin, [1, 2]);
1239 /// assert_eq!(*end, [3, 4]);
1240 /// begin.copy_from_slice(&[4, 3]);
1241 /// end.copy_from_slice(&[2, 1]);
1243 #[unstable(feature = "refcell_map_split", issue = "51476")]
1245 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1246 orig: RefMut<'b, T>, f: F
1247 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1248 where F: FnOnce(&mut T) -> (&mut U, &mut V)
1250 let (a, b) = f(orig.value);
1251 let borrow = orig.borrow.clone();
1252 (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
1256 struct BorrowRefMut<'b> {
1257 borrow: &'b Cell<BorrowFlag>,
1260 impl<'b> Drop for BorrowRefMut<'b> {
1262 fn drop(&mut self) {
1263 let borrow = self.borrow.get();
1264 debug_assert!(is_writing(borrow));
1265 self.borrow.set(borrow + 1);
1269 impl<'b> BorrowRefMut<'b> {
1271 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1272 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1273 // mutable reference, and so there must currently be no existing
1274 // references. Thus, while clone increments the mutable refcount, here
1275 // we explicitly only allow going from UNUSED to UNUSED - 1.
1276 match borrow.get() {
1278 borrow.set(UNUSED - 1);
1279 Some(BorrowRefMut { borrow: borrow })
1285 // Clone a `BorrowRefMut`.
1287 // This is only valid if each `BorrowRefMut` is used to track a mutable
1288 // reference to a distinct, nonoverlapping range of the original object.
1289 // This isn't in a Clone impl so that code doesn't call this implicitly.
1291 fn clone(&self) -> BorrowRefMut<'b> {
1292 let borrow = self.borrow.get();
1293 debug_assert!(is_writing(borrow));
1294 // Prevent the borrow counter from underflowing.
1295 assert!(borrow != isize::min_value());
1296 self.borrow.set(borrow - 1);
1297 BorrowRefMut { borrow: self.borrow }
1301 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1303 /// See the [module-level documentation](index.html) for more.
1304 #[stable(feature = "rust1", since = "1.0.0")]
1305 pub struct RefMut<'b, T: ?Sized + 'b> {
1307 borrow: BorrowRefMut<'b>,
1310 #[stable(feature = "rust1", since = "1.0.0")]
1311 impl<'b, T: ?Sized> Deref for RefMut<'b, T> {
1315 fn deref(&self) -> &T {
1320 #[stable(feature = "rust1", since = "1.0.0")]
1321 impl<'b, T: ?Sized> DerefMut for RefMut<'b, T> {
1323 fn deref_mut(&mut self) -> &mut T {
1328 #[unstable(feature = "coerce_unsized", issue = "27732")]
1329 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1331 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1332 impl<'a, T: ?Sized + fmt::Display> fmt::Display for RefMut<'a, T> {
1333 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1338 /// The core primitive for interior mutability in Rust.
1340 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1341 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1342 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1343 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1345 /// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are
1346 /// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably
1347 /// aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language
1348 /// feature to work around this restriction. All other types that allow internal mutability, such as
1349 /// `Cell<T>` and `RefCell<T>`, use `UnsafeCell` to wrap their internal data.
1351 /// The `UnsafeCell` API itself is technically very simple: it gives you a raw pointer `*mut T` to
1352 /// its contents. It is up to _you_ as the abstraction designer to use that raw pointer correctly.
1354 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1356 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1357 /// reference) that is accessible by safe code (for example, because you returned it),
1358 /// then you must not access the data in any way that contradicts that reference for the
1359 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1360 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1361 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1362 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1363 /// safe code, then you must not access the data within the `UnsafeCell` until that
1364 /// reference expires.
1366 /// - At all times, you must avoid data races. If multiple threads have access to
1367 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1368 /// accesses (or use atomics).
1370 /// To assist with proper design, the following scenarios are explicitly declared legal
1371 /// for single-threaded code:
1373 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1374 /// references, but not with a `&mut T`
1376 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1377 /// co-exist with it. A `&mut T` must always be unique.
1379 /// Note that while mutating or mutably aliasing the contents of an `&UnsafeCell<T>` is
1380 /// okay (provided you enforce the invariants some other way), it is still undefined behavior
1381 /// to have multiple `&mut UnsafeCell<T>` aliases.
1386 /// use std::cell::UnsafeCell;
1387 /// use std::marker::Sync;
1389 /// # #[allow(dead_code)]
1390 /// struct NotThreadSafe<T> {
1391 /// value: UnsafeCell<T>,
1394 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
1396 #[lang = "unsafe_cell"]
1397 #[stable(feature = "rust1", since = "1.0.0")]
1398 pub struct UnsafeCell<T: ?Sized> {
1402 #[stable(feature = "rust1", since = "1.0.0")]
1403 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1405 impl<T> UnsafeCell<T> {
1406 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1409 /// All access to the inner value through methods is `unsafe`.
1414 /// use std::cell::UnsafeCell;
1416 /// let uc = UnsafeCell::new(5);
1418 #[stable(feature = "rust1", since = "1.0.0")]
1420 pub const fn new(value: T) -> UnsafeCell<T> {
1421 UnsafeCell { value: value }
1424 /// Unwraps the value.
1429 /// use std::cell::UnsafeCell;
1431 /// let uc = UnsafeCell::new(5);
1433 /// let five = uc.into_inner();
1436 #[stable(feature = "rust1", since = "1.0.0")]
1437 pub fn into_inner(self) -> T {
1442 impl<T: ?Sized> UnsafeCell<T> {
1443 /// Gets a mutable pointer to the wrapped value.
1445 /// This can be cast to a pointer of any kind.
1446 /// Ensure that the access is unique (no active references, mutable or not)
1447 /// when casting to `&mut T`, and ensure that there are no mutations
1448 /// or mutable aliases going on when casting to `&T`
1453 /// use std::cell::UnsafeCell;
1455 /// let uc = UnsafeCell::new(5);
1457 /// let five = uc.get();
1460 #[stable(feature = "rust1", since = "1.0.0")]
1461 pub fn get(&self) -> *mut T {
1462 &self.value as *const T as *mut T
1466 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1467 impl<T: Default> Default for UnsafeCell<T> {
1468 /// Creates an `UnsafeCell`, with the `Default` value for T.
1469 fn default() -> UnsafeCell<T> {
1470 UnsafeCell::new(Default::default())
1474 #[stable(feature = "cell_from", since = "1.12.0")]
1475 impl<T> From<T> for UnsafeCell<T> {
1476 fn from(t: T) -> UnsafeCell<T> {
1481 #[unstable(feature = "coerce_unsized", issue = "27732")]
1482 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1485 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1486 let _: UnsafeCell<&Send> = a;
1487 let _: Cell<&Send> = b;
1488 let _: RefCell<&Send> = c;